CIRPASS â€” Doing Science on Gemini South

August 27, 2002

Exploring mass sub-structure in the Einstein Cross lensing galaxy

A gravitational lens is created when a massive foreground object (a large intervening galaxy in this case of the Einstein Cross) bends and amplifies the light from a far more distant object (a quasar in this case) directly behind it. In certain circumstances, the background object may be distorted or even multiply-imaged into two or four images. The Einstein Cross (Q2237+0305) is such a 4-image lens, where the bulge of a nearby spiral galaxy is lensing a very distant quasar. In the case of the Einstein Cross, the more distant quasar appears much brighter than the intervening galaxy and four images of the same quasar are formed. Without the intervening galaxy, the quasar image would be a single object of much fainter magnitude.

Spectrum of one of the QSO sub-images

The goal of the CIRPASS near-infrared spectroscopy program on the Einstein Cross is to explore mass substructures of 10^4 to 10^7 solar masses that may be present in the dark matter halo of the lensing galaxy responsible for the multiple quasar images. From comparative modeling of a smooth-potential case versus a halo with substructure the CIRPASS investigators are predicting different gravitational signatures. Simultaneous spectra obtained across the whole object are critical. CIRPASS allows to measure simultaneously the relative intensities (with respect to the Narrow Line Region [OIII] lines) and equivalent widths (with respect to the underlying continuum) of the Broad Line Region as sampled by the H-beta line. The observations obtained on Gemini South will allow to measure the filling factor of substructures, and to estimate the level of microlensing in progress. By use of the IFU, all images are measured simultaneously and without the problematic effects of slit losses and differential refraction, endemic to longslit observations.

The Gemini Observatory is an international collaboration with two identical 8-meter telescopes. The Frederick C. Gillett Gemini Telescope is located on Maunakea, Hawai'i (Gemini North) and the other telescope on Cerro Pachón in central Chile (Gemini South); together the twin telescopes provide full coverage over both hemispheres of the sky. The telescopes incorporate technologies that allow large, relatively thin mirrors, under active control, to collect and focus both visible and infrared radiation from space.

The Gemini Observatory provides the astronomical communities in five partner countries with state-of-the-art astronomical facilities that allocate observing time in proportion to each country's contribution. In addition to financial support, each country also contributes significant scientific and technical resources. The national research agencies that form the Gemini partnership include: the US National Science Foundation (NSF), the Canadian National Research Council (NRC), the Argentinean Ministerio de Ciencia, Tecnología e Innovación Productiva, the Brazilian Ministério da Ciência, Tecnologia e Inovação and the Chilean Comisión Nacional de Investigación Científica y Tecnológica (CONICYT). The observatory is managed by the Association of Universities for Research in Astronomy, Inc. (AURA) under a cooperative agreement with the NSF. The NSF also serves as the executive agency for the international partnership.